Confused by some words

Question

In Three Days to See, I read

Now and then I have tested my seeing friends to discover what they see. Recently I was visited by a very good friend who had just returned from a long walk in the woods, and I asked her what she had observed. "Nothing in particular," she replied. I might have been incredulous had I not been accustomed to such responses, for long ago I became convinced that the seeing see little.

"incredulous" means "suspicious", so:

1. "I" didn't believe that nothing in particular can be seen from a long walk in the woods.
2. However, after "I" have been accustomed to such responses, "I" then believed.

However, the sentence right after confused me:

for long ago I became convinced that the seeing see little

I thought it means that "I" was convinced that the seeing see little long ago, which contradicted with the fact above that "I" didn't believe that some time ago. If "I" have already convinced long ago, "I" just didn't need others' responses to learn that the seeing see little.

Answer 1

You have all the word meanings correct but you are missing the construction the author uses.

The author says they "might have been incredulous had they not been accustomed to such responses." The author would be surprised at how someone could say they saw nothing in particular on walk, and likely was surprised the first few times they had that conversation with someone—but now they are no longer surprised. The clause at the end of the sentence explains why they are not surprised: because they became convinced long ago that "the seeing see little."

The two events (the author being surprised, and the author becoming accustomed and therefore not being surprised) both happened in the past. But the surprise happened first, and only after that did the author become accustomed to the situation.